The present invention relates to a chip and method for the production of wood pulp, especially for the production of paper.
Wood chips for the pulp and paper industry have been produced for years from equipment known in the art as "chippers" or, occasionally, "choppers." This equipment employs a knife for cutting repeatedly into a log or other bulk piece of wood to produce chips having relatively small dimensions of length, width and thickness.
The chips are processed by "cooking" them in a digester at temperatures of about 170-180 degrees centigrade in a sodium hydroxide or sodium bisulfide "liquor" to dissolve the lignins and other binders in the chips and leave behind the cellulose fibers. The liquor diffuses into the chips at a predetermined rate.
It has been estimated that an increase in digester yield of just 1% provides a savings of about $1 million per year per digester. This yield is determined by a number of factors. One of these is chip uniformity. Uniform chip shapes and sizes provide for greater packing density. In the digester, this provides a greater amount of cellulose from a given batch of chips. Uniform chip thickness is particularly important to digester yield. This dimension is smaller than the length and width of the chips, and controls the time required for the liquor to diffuse sufficiently into the chip to dissolve the lignin. Chips that are thicker than the target thickness spend too little time in the digester for removal of all of the lignin, and chips that are thinner than the target thickness are overcooked in the digester so that the liquor attacks and degrades the cellulose fibers themselves.
The absolute shape and size of the chips are also important factors in the efficient conversion of wood chips to cellulose. It is desirable that the chips be thin to minimize the difference in time that fibers in the interior of the chip and fibers on the exterior of the chip are cooked. On the other hand, chipping the wood so as to produce very thin chips mechanically damages a greater percentage of the total fiber in the chips. Accordingly, there has been determined in the pulping industry an acceptable chip thickness lying within the range of about 1 mm to about 8-10 mm, with the optimum chip thickness being about 4-5 mm.
Chip shape is also an important contributor to efficient cellulose production. Conventional chip shapes result from forming processes that bruise and damage the wood fibers. As a response to this problem, Altosaar, U.S. Pat. No. 3,304,970 proposes a chip and process for forming the chip wherein the main or larger faces of the chip are produced by cutting substantially parallel to the grain while the two side edges are cut across and at an angle to the grain, with the remaining end surfaces being formed by splitting or cleaving along the grain. However, in cutting the wood across the grain at an angle to form the side edges, an increased cut surface area results. In contrast with cleaving the wood along the grain, cutting the fibers damages the ends thereof, and cutting them at an angle exposes more of the fibers to such damage.
The magnitude of and variation in the thicknesses of the chips is of primary importance to digester yield, while variation in the lengths of the chips is less important and the magnitude of and variation in the widths of the chips is generally considered to have minor or negligible importance.
The defining characteristic of chipper equipment is that it is adapted to cut wood mainly across the grain. The chips so produced have a length that is relatively well controlled by the depth of penetration of the knife into the wood. On the other hand, they have a thickness and width that are not well controlled. The thickness, particularly, depends on a number of factors, including the type of wood and its moisture content, whether the wood is frozen, and the cutting geometry. Chip thickness can be controlled somewhat by controlling chip length; however, the resulting chips are distributed about the desired mean chip thickness so that a large number of the chips exceed the tolerable range. Accordingly, an expensive and inefficient process of sorting reject chips and reworking them into an acceptable form is required.
Another type of equipment, known as the waferizer or strander, has been employed to produce wafers, strands or flakes of wood ("flakes") for the production of waferboard or oriented strand board ("OSB"). The waferizer is similar in principle to the chipper, except that it cuts the wood substantially parallel to the grain to produce flakes having a very small thickness, e.g., about 0.025", and relatively long lengths of about 4" to 5". In the waferizer, the thickness corresponds to the amount the apparatus cuts into the wood. Since this is a relatively small amount in the waferizer in comparison with the chipper, the waferizer is provided with a relatively low power so that practical examples are inadequate for producing chips for pulp.
Accordingly, there is a need for a chip and method for the production of wood pulp that optimizes digester yield and, accordingly, the yield and efficiency of the entire pulping process, by improving control of variations in the width and length of the chips, and by optimizing the shape of the chips.